1. Field of the Invention
The present invention relates to a flat type cathode ray tube for a color television receiver set or a computer terminal display, and a color image display apparatus utilizing the same.
2. Description of the Prior Art
U.S. patent application Ser. No. 748,833 by the assignee of the present application discloses a flat type cathode ray tube shown in FIG. 1. In the actual structure, electrodes are contained in a vacuum enclosure (glass vessel), however, the vacuum enclosure has been excluded in FIG. 1 to clearly show the internal electrodes. A horizontal axis H and a vertical axis V are shown on a faceplate to define horizontal and vertical directions of a display screen on which an image or characters is displayed.
A plurality of line cathodes 10 which extend vertically and are made of tangusten wires with an oxide formed on the surfaces thereof are arranged horizontally and independently at equal spatial intervals. The number of the line cathodes 10 and the spacing thereof are arbitrary. Assuming that the display screen size is 10 inches, the horizontal spacing is approximately 10 mm, and twenty line cathodes having a vertical length of approximately 160 mm are arranged. Vertical scan electrodes 12, which are electrically separated from each other and extend horizontally, are arranged at constant vertical spatial intervals on an insulating support 11 which are disposed closely to the line cathodes 10 and on one side of the line cathodes 10 opposite to the other side thereof facing a faceplate 9. If a conventional television image is to be displayed, the vertical scan electrodes 12 have the same number of independent electrodes as the number of horizontal scan lines (about 480 lines for the NTSC system). A first grid electrode (G1) 13 comprising planar electrodes, each of which has openings at positions corresponding to each one of the line cathodes 10 and which are separated in correspondence with the respective line cathodes 10, for effecting the beam-modulation by applying video signals to the individual planar electrodes, a second grid electrode (G2) 14 and a third grid electrode (G3) 15, which have similar openings to those of the electrode (G1) 13 but are not separated electrically and horizontally, are arranged sequentially between the line cathodes 10 and the faceplate 9 in the direction from the line cathodes 10 to the faceplate 9. Then, a fourth grid electrode (G4) 16 having the same openings or horizontally broader openings as compared with the openings of the electrodes (G2) 14 and (G3) 15 is arranged. Next, horizontal focusing electrodes 17 and horizontal deflection electrodes 18, both of which are formed on a surface of an insulating support 19 by the plating or vapor-depositing process, are arranged symmetrically with respect to the electron beam paths at the same horizontal intervals as that of the line cathodes 10. A light emitting layer comprising a phosphor screen 7 and a metal back electrode 8 is formed on an inner surface of the faceplate 9. In a color display apparatus, the phosphor screen 7 is formed by arranging red (R), green (G) and blue (B) phosphors horizontally and sequentially in stripes or dots.
Referring to FIGS. 2 and 3A and 3B, the operation of the above-mentioned color cathode ray tube is explained. The line cathodes 10 are heated by causing electric currents to flow therethrough. Voltages substantially equal to the voltage of the line cathodes 10 are applied to the electrode (G1) 13 and the vertical scan electrodes 12. A voltage (100-300 volts) higher than the electric potential of the cathodes 10 is applied to the electrode (G2) 14 so that beams are directed from the cathodes 10 to the electrodes (G1) 13 and (G2) 14 and pass through the openings of the respective electrodes. In order to control the quantity of the electron beams passing through the openings of the electrodes G1 and G2, the voltage of the electrode (G1) 13 is varied. The electron beams which have passed through the openings of the electrode (G2) 14 further pass through the electrode (G3) 15, electrode (G4) 16 and horizontal focusing electrode 17. Predetermined voltages are applied to those electrodes so that the electron beams are focused to a small spot on the phosphor plane. The vertical electron beam focusing is carried out by an electrostatic lens formed at an exit of the openings of the electrode (G4) 16, and the horizontal electron beam focusing is carried out by an electrostatic lens formed between the horizontal focusing electrode 17 and the horizontal deflection electrode 18. The electron beam which passed through the horizontal focusing electrode 17 is horizontally deflected by a predetermined width due to a step deflection voltage or a sawtooth wave of a horizontal scan period applied to the horizontal deflection electrode 18, and it stimulates the phosphor 7 to produce a light image. In order to produce a color image, a modulation signal for a color corresponding to the color phosphor to which the electron beam is directed is applied to the electrode (G1) 13.
The vertical scan is explained with reference to FIGS. 3A and 3B. The generation of electrons from the line cathodes 10 is controlled by controlling the voltage of the vertical scan electrode 12 in a way such that the electrostatic field around the line cathodes 10 is made positive or negative with respect to the electric potential of the line cathodes 10. If the distance between the line cathodes 10 and the vertical scan electrodes 12 is small, a voltage for controlling ON or OFF of electron beams from the cathodes 10 may be low. When an interlaced system is adopted, in a first field, a signal is applied to a vertical scan electrode 12A to generate (ON) an electron beam only for one horizontal scan period (1H), and for the next 1H period, a signal is applied to a vertical scan electrode 12C to generate (ON) an electron beam, and then in a similar manner, signals to generate electron beams only for the 1H period are sequentially applied to every other vertical scan electrode. When a signal is applied to the bottom electrode 12X, the vertical scan of the first one field is completed. In the second field, a signal for generating an electron beam only for the 1H period is sequentially applied starting from the electrode 12B to the final electrode 12Y, whereby the vertical scan of one frame is completed.
With this arrangement, however, it is difficult to maintain a precise electron beam spot size and a precise electron beam incident position on the phosphor screen for the respective beams in the whole electron beam teansmission range from electron guns to the phosphor plane, and the deflection sensitivity is low.
Further, it is necessary that the number of the vertical scan electrodes, which are arranged behind the line cathodes, corresponding to the number of horizontal scan lines is approximately 480 for the conventional television system, or one half thereof, that is, 240 for a system in which the beam positions are vertically shifted in the interlaced operation. It is desirable to make the number of the vertical scan electrodes as small as possible in view of not only the manufacture of the electrodes but also the connection of the vertical scan electrodes with the drive circuits and the number of the drive circuits.
Besides, in order for a color image to be exactly displayed on the above-mentioned flat type cathode ray tube, a modulation signal for a color corresponding to the color phosphor to which the electron beam is directed should be applied to the eleotrode Gl. However, since this cathode ray tube has no such function, if the horizontal deflection width varies or the linearity of the horizontal deflection is insufficient, the hue of the color image varies. Further, if the spatial interval of the horizontal deflection electrodes becomes nonuniform in the manufacture of the cathode ray tube, the hue of the color image also varies at every horizontal block.